About two months after the 9/11 terrorist attacks, Michael Vincent attended a Southern California conference on drunken driving hosted by the National Highway Safety Administration. In one presentation, the speaker began by pointing out that 15,000 Americans were dying in alcohol-related driving accidents in an average year.

That equates to about 50 jumbo jets full of people – or five times the number of Americans killed on 9/11.

For the first time, his emotions raw following the terrorist attacks, Vincent was hit by the enormity of how many lives are lost in the United States at the hands of impaired drivers. As an executive for Immunalysis, a manufacturer of drug-testing equipment, he decided to begin research on a new generation of drug-detection equipment. His company then developed an accurate and speedy way for law enforcement to detect the presence of drugs in a driver’s saliva, as opposed to depending on blood tests.

Impaired-driving deaths don’t change “the headlines or our psyche,” Vincent said. Instead, the losses are quieter, tallied one by one in isolated incidents.

To use a common idiom, drunken driving is the devil we know. Decades of advertising and education have imparted that message to us from an early age. Those campaigns have helped decrease the number of drunken-driving deaths to about 10,000 yearly.

Drug-impaired driving has long been the less visible evil, made worse by growing rates of prescription-drug abuse as well as sustained, and in some regions rising, levels of illicit drug use. According to the federal Centers for Disease Control and Prevention, drugs other than alcohol are involved in about 18 percent of motor vehicle deaths. Often, drugs are used in combination with alcohol, a particularly dangerous combination.

In their efforts to keep the streets safe, law enforcement officers are grappling with a new set of challenges in identifying and prosecuting what is known as DUID, or driving under the influence of drugs. On the street and in the courtroom, there are few or no standards to judge a driver’s level of impairment. Legal codes are often inadequate and outdated to deal with drugged driving.

With the great variety of prescription and illicit drugs in circulation, the most basic decision – judging whether, and to what extent, a driver is intoxicated – is no easy task.

At a mid-August DUI checkpoint in Del Paso Heights, Sgt. Christian Prince of the Sacramento Police Department explained it to me most simply: “There’s no 0.08.”

The Sacramento Police Department made 1,436 DUI arrests in 2012, of which 82 were for drug-impaired driving, Prince said in an email. So far this year, the department has made 830 DUI arrests and 77 for drugged driving, he said.

In the case of drunken driving, law enforcement may ask a driver to take a Breathalyzer test for a quick, accurate read on their blood-alcohol content to determine whether they’ve breached the legal limit of 0.08. The officer knows what to look for, and the result is a yes-or-no determination.

“With alcohol levels, there’s a fairly direct one-to-one relationship with impairment. The higher the level is, the more impaired you are. It doesn’t hold true with most drugs,” said Michael Walsh, formerly the head of the President’s Drug Advisory Council under the first Bush administration and president of the Walsh Group, a consulting firm with a focus on substance abuse.

Take marijuana as an example. Short of a cloud of smoke pouring from a vehicle’s windows, it can be almost impossible for officers to verify whether an individual is high on marijuana. Physical signs, such as red, puffy eyes or changes in speech and behavior, vary depending on how much was used and the user’s tolerance.

An individual’s metabolism also affects the available evidence of intoxication. Within the first 15 minutes of smoking pot, traces of the drug in the blood system reach their highest level, Walsh said. After two hours, concentrations of the drug drop, but the user’s impairment might be just as significant, he added.

Eighteen states have passed zero-tolerance policies that prohibit any trace of drugs in drivers, according to the Governors Highway Safety Association, a nonprofit advocacy organization that addresses issues of traffic safety. Colorado and Washington, which legalized marijuana for recreational use last year, have set legal thresholds for the presence of THC, the psychoactive component of the drug, in nanograms per milliliter.

Currently, California is not one of the zero-tolerance states; a bill to change that didn’t get very far in the legislative session that ended Thursday. Law enforcement groups that opposed Proposition 19, the unsuccessful ballot measure in 2010 to legalize marijuana for personal use, warned that it would increase the number of stoned drivers. They also criticized Proposition 19 because it did not define what would constitute driving while impaired by pot.

California, New York and Hawaii are the only states with statutes that distinguish among drunken driving, drug-impaired driving and driving under the influence of both alcohol and drugs, according to the Governors Highway Safety Association.

In January, two new sections will become active in California’s vehicle code, stating explicitly that it is illegal for a person “under the influence of any drug” or “the combined influence of any alcoholic beverage and drug” to get behind the wheel. In doing so, officials will be better equipped to tally the number of drugged drivers as well as drivers impaired by both drugs and alcohol.

But even with benchmarks on the books, they would still be almost unenforceable without the proper equipment.

Roadside drug tests are not widely used. The newest systems, which test saliva, are still undergoing tests, and many legislators and district attorneys haven’t been sold on their quality, Vincent said. Traditional drug testing involves blood or urine samples.

Drug-impaired driving, like its better-known counterpart, is a problem with only imperfect solutions. But each year, it inflicts a deadly toll. At what point will we seriously confront it?